Quite frequently there's talk about adding units to the National Park System. But where do you begin, and where do you draw a line? Well, it turns out the National Park Service has a cheat-sheet of sorts that touches on the timelines of studies into proposed units of the system.

Presently, though, there are more gaps in the document than firm deadlines. The latest version of this document -- September 7, 2009 -- touches on 45 authorized studies, ranging from 27 potential additions to the park system and eight potential National Heritage Areas to five possible additions to the National Trails System, and five additions to the National Wild and Scenic River System.

Proposed units to the park system include:

* A Coltsville Industrial Historic District in Connecticut, which draws its fame, in part, from being the hometown of the inventor of the Colt revolver. This study is scheduled to reach Congress by March 2010.

The Coltsville Study area is a 260-acre industrial area in Hartford, CT. It was recognized as the Colt Industrial National Register District in 1976. Within this historic district is inventor Samuel Colt's house Armsmear, which was designated as a National Historic Landmark in 1966. Also in Coltsville are 11 historic industrial buildings located on 17 acres. In 2003, a private developer Colt Gateway LLC (Homes for America Holdings, Inc.) acquired these buildings and is redeveloping most of them for commercial, residential, and office tenants. The developer has offered to make available appropriate space to create a site for National Park Service use. Also within the study area, but in separate ownership, are former Colt worker housing units, a church and parish house built by Samuel Colt's wife Elizabeth, and several other buildings associated with Colt history. The study area borders Interstate-91, which parallels the Connecticut River, and is close to the Hartford central business district, where the State Capitol, Museum of Connecticut History, and the Wadsworth Atheneum are located. The Museum of Connecticut History and the Wadsworth Atheneum are major repositories of Colt-related artifacts and archives.

* A Michigan Maritime Sites unit in the state of Michigan. This would be no small project, as the legislation calling for the Park Service to study such an addition says "(t)he term `maritime heritage resource' includes lighthouses, lifesaving and coast guard stations, maritime museums, historic ships and boats, marine sanctuaries and preserves, fisheries and hatcheries, locks and ports, ore docks, piers and breakwaters, marinas, resort communities (such as Bay View and Epworth Heights), cruises, performing artists that specialize in maritime culture, interpretive and educational programs and events, museums with significant maritime collections, maritime art galleries, maritime communities, and maritime festivals." There is no targeted deadline for a study on this addition to be completed.

* Castle Nugent Farms on St. Croix, Virgin Islands. A study on this proposed addition is expected to reach Congress by June 2010.

The Castle Nugent Farms study area encompasses an area that contains the remnants of onetime smaller cotton, sugar, indigo and cattle plantations. Located on the arid southeastern shore, about three miles southeast of Christiansted, the study area consists of approximately 2,900 acres of hilly and rolling terrain that contains mostly a mixture of dry forest, native vegetation, and rangeland and five miles of coastline that extends out from the shoreline to the three-mile territorial limit. The lands contained within the area are referred to mostly by the names of the original Danish plantations (also referred to as estates) that were established there.

The estate known as Castle Nugent is the core 1,400 acres of the entire 2,900-acre study area. The estates formerly known as Laprey Valley, The Springs, and Munster were included within Castle Nugent holdings in the late eighteenth and nineteenth centuries. The estates Fareham, Petronella, and Longford were added when Castle Nugent was purchased by Howard and Mary Wall in 1951. The overall study area is comprised of mostly open land that enjoys impressive views to the sea and to distant parts of the island. The land slopes from the 750-foot ridgeline down to the sea, fronting a coastline of cobble beaches and small crescent bays. One of the largest and healthiest fringing coral reef systems in the Virgin Islands extends from the shoreline to only a few hundred feet offshore within U.S. Virgin Islands territorial waters. The study area also contains a variety of natural communities and habitats that represent an exceptional diversity of ecological components that have been little disturbed. Of particular note is an area known as Great Pond Bay, a fringe, reef-protected lagoon that is a mecca for several species of birds and contains the most substantial black mangrove stands left in the Virgin Islands.

* Newtonia Civil War Battlefields in Missouri. According to a report presented to the Senate and Energy and Natural Resources Committee in June 2007, "Newtonia, Missouri was the scene of two significant battles in the Civil War, one on September 30, 1862, and the other on October 28, 1864. The battles were fought in and around the town of Newtonia, an area that today retains much of its character from almost a century and a half ago. The September 30, 1862, battle involved Native American soldiers directly fighting each other, and the October 28, 1864, battle was the last Civil War battle fought within the borders of the State of Missouri and the culmination of Confederate Major General Sterling Price’s 1864 Missouri Expedition. Newtonia was important as a centralized location to lead mines and the communities of Mt. Vernon (which housed a Union garrison), Sarcoxie, and Neosho, making it a priority for both sides seeking control of the Spring River and its fertile valley." A study on this area began this year, though there's no deadline for its submittal to Congress.

* Butterfield Overland Trail, which was the country's longest stagecoach route, running from Tipton, Missouri, to San Francisco. A study on this proposed addition was authorized this year, but has not yet begun.

* Fort San Geronimo in Puerto Rico. Built in 1609, the tiny fort in 1983 was added to the National Register of Historic Places. A study on this proposed addition to the park system was authorized in 2009, but has not yet begun.

* The Harriet Beecher Stowe House in Connecticut. Stowe, of course, wrote Uncle Tom's Cabin, which urged freedom for slaves. A study on this proposed addition was authorized this year, but has not yet begun.

* San Gabriel Watershed and Mountains. A study on this proposed addition was authorized in 2003. It is expected to be delivered to Congress in May 2010.

The study area covers more than 1,000 square miles, or 700,000 acres. The United States Forest Service manages approximately 2/3 of the study area in the San Gabriel Mountains as part of the Angeles National Forest. Most of the study area is located in Los Angeles County, with small portions in Orange and San Bernardino counties. While much of the study area lies within the San Gabriel River watershed, portions of the Los Angeles River, the Santa Clara River and the Antelope Valley watersheds are also included.

The diverse study area landscape contains mountains, valleys, wildlands, and urban areas. The San Gabriel Mountains foothills function as the urban/wildlife interface, and provide wildlife connections to river corridors. Although portions of the San Gabriel River and its tributaries have been altered for flood protection and water conservation, these urbanized channels still serve as habitat for wildlife and often provide opportunities for recreation.

Conspicuously absent from those areas now being studied, save for the San Gabriel Watershed and Mountains, are expansive landscapes. Of course, that in large part is due to a lack of congressional direction to the Park Service to study proposed additions.